The present invention relates to polymer coated high strength steel articles that exhibit excellent corrosion resistance and simultaneously exhibit low hydrogen embrittlement and reembrittlement characteristics, and methods for making the same. The present invention also relates to polymer coated articles exhibiting excellent long term corrosion resistance.
High strength structural materials such as high strength steels do not in their bare form generally offer desirable corrosion resistance properties. Consequently, techniques have been developed for improving the corrosion resistance of these high strength materials. Often, a metal or metal alloy is plated onto the high strength steel to enhance its environmental stability.
It has been found, however, that when metallic coatings are plated onto high strength materials, hydrogen is often co-deposited at the surface of the metal substrate. The presence of the hydrogen has detrimental effects on various physical and mechanical properties of the high strength materials. For example, once hydrogen enters a high strength steel substrate, the metal substrate loses its ductility, and depending upon the level of hydrogen present in the substrate, can suffer brittle failure when subjected to stress. This much studied, but vaguely understood, phenomenon is referred to as "hydrogen embrittlement".
The hydrogen embrittlement problem associated with plating of metallic coatings onto a high strength steel substrate has for years been remedied by the use of a corrosion resistant cadmium-titanium coating. Once the cadmium-titanium coating has been applied to the substrate, the coated article is baked at an elevated temperature for an extended period of time to drive any hydrogen from the substrate. This procedure has been found to eliminate the hydrogen embrittlement problem. More recently, the use of an electroplated zinc-nickel alloy has been recommended for replacement of the old cadmium-titanium process. Use of the zinc-nickel also requires baking following the plating process to eliminate hydrogen from the substrate.
More recently it has been noted that hydrogen may enter a metallic coating and again find its way to the metal substrate while the plated high strength steel is in use. This hydrogen migration especially occurs where the plated high strength steel is subjected to a corrosive environment, particularly a saline environment or where a plating deposit is scratched or otherwise mechanically damaged. If a sufficient amount of hydrogen accumulates within the substrate, the hydrogen embrittlement problem can reappear. This phenomenon is referred to as "hydrogen reembrittlement". Hydrogen reembrittlement can again result in brittle failure of the high strength steel parts when subjected to stress. By carefully controlling the zinc-nickel plating process referenced above, the hydrogen reembrittlement problem can be minimized.